Rowanberry Nectar—The Effect of Preparation Method, Sweetener Addition, and Storage Condition on Bioactive Compounds

Rowanberries (Sorbus aucuparia) are valued for their high content of bioactive compounds. This study examined the effects of sweeteners (sucrose, xylitol, erythritol, steviol glycosides), fruit pulp preparation (fresh vs. steamed), and storage conditions (4 °C and 30 °C, 3 months) on the composition of rowanberry nectars. Polyphenols were quantified using LC-PDA-QTOF/MS and UPLC-PDA-FL, and carotenoids, organic acids, antioxidant capacity (FRAP), and physicochemical properties were also determined. Steaming increased total polyphenol levels in nectars by 13–52%, with the highest values observed in formulations containing steviol glycosides (up to 1833 mg/100 mL). Changes in carotenoid content during storage varied depending on the sweetener type. In steamed nectars with erythritol stored at 4 °C, carotenoid levels remained close to those measured in the corresponding unsweetened steamed sample. Storage influenced turbidity and viscosity in all variants, with the largest viscosity increases recorded in stevia- and erythritol-sweetened nectars. Overall, the combined effects of fruit preparation, sweetener type, and storage determined the final composition and stability of rowanberry nectars.